1. Field of the Invention
The present invention relates to a variable focal position optical system and to a light beam scanning apparatus, and in particular, to a variable focal position optical system in which the focal position of an incident light beam is varied, and to a light beam scanning apparatus which includes the variable focal position optical system.
2. Description of the Related Art
Laser computer output microfilmers (laser COMs) are known as devices for recording information such as characters or images onto a recording material by a light beam. In the laser COM, for example, a laser beam is scanned on the basis of information outputted from a computer, and information such as characters is directly recorded onto a recording material such as a microfilm (Japanese Patent Application Laid-Open No. 55-67722). In this type of laser beam recording device, after the laser beam is scanned by scanning means such as a polygon mirror, a galvanometer mirror or the like, the laser beam is imaged onto the recording surface of a recording material by a scanning lens such as an f.theta. lens. Characters, an image or the like are thereby recorded onto the recording material.
Generally, curvature of field remains at the scanning lens, and even if an ideal (i.e., no wave aberration) laser beam is incident on the scanning lens, the beam waist position does not correspond to the recording surface of the recording material. Further, due to reasons such as the curvature of the scanning lens being non-uniform, the focal position (beam waist position) of the laser beam which is scanned by the scanning means and passes through the scanning lens does not always correspond to the recording surface (so-called "defocus"). Especially in a case in which a semiconductor laser has an astigmatism and the laser beam irradiated from the semiconductor laser is used as the light beam, there is deviation (so-called astigmatic difference) between the beam waist position in the main scanning direction of the laser beam and the beam waist position in the subscanning direction due to the astigmatism of the semiconductor laser and the optical system.
Distortion of the shape of the laser beam illuminated onto the recording surface occurs due to the defocus and astigmatic difference, and deficiencies such as lack of clarity are generated in portions of the characters, image or the like recorded on the recording surface. In recent years, the demands made on light beam recording devices which can record images of larger sizes have increased. Deterioration of image quality which is caused by the above-mentioned phenomena and which accompanies the increase in sizes of recorded images has come to be a significant problem.
In order to solve this problem, Japanese Patent Application Laid-Open No. 3-290610 uses a "cylindrical lens whose focal length can be varied by electric operation". By varying the focal length of the cylindrical lens, the beam waist position in the subscanning direction of the laser beam is varied. Curvature, with respect to the recording surface, of the image surface in the subscanning direction, which curvature corresponds to the locus of the beam waist position in the subscanning direction of the laser beam (hereinafter referred to as "curvature of field") is thereby corrected.
In the above-mentioned publication, the beam waist position is changed by varying the focal length of the cylindrical lens. However, when the focal length is varied in this way, a drawback arises in that the beam diameter .omega.' at the beam waist position of the laser beam exiting from the cylindrical lens (hereinafter, "beam waist diameter") fluctuates, as is clear from following formula (1): ##EQU1## wherein .omega.: beam diameter of the laser beam incident on the cylindrical lens
.lambda.: wavelength of the incident light beam PA1 f: focal length PA1 K: constant.
Accordingly, even if the focal length f of the cylindrical lens is varied such that the beam waist position of the laser beam always corresponds to the recording surface, the beam waist diameter of the laser beam varies as the focal length f of the cylindrical lens varies. Therefore, drawbacks still arise such as portions of the characters, images or the like recorded on the recording surface are unclear.